Can you recommend a decent online or software calculator? I'm looking for an online or software calculator that can show me the history of items I typed in, much like an expensive Ti calculator. 
Can you recommend any?
 A: Lots of people like to use InstaCalc which lets you do unit conversions and store intermediate calculations in variables.
A: I use R these days. It was built to be a calculator and does the job well. Its syntax might be a bit strange but it allows you to do a lot with little typing.
A: Try sage.  It's like python (it is actually written in python, and uses something similar to a python shell), but with a vastly larger set of built-in mathematical operations.
A: For desktop software, I use SpeedCrunch. Has a history, lots of mathematical functions, supports variable, etc.
A: Microsoft Math Solver looks like TI and shows the history of items.
A: In case you happen to be an Emacs user, the Emacs Calc package is a very good programmable stack-based calculator/mini-CAS.
("M-x calc" to start, then "i" to open the manual in info format.)
A: This might be a little more basic than you are wanting but this is web-based "Desmos" - https://www.desmos.com/ does a good pretty good job graphing things.
A: Essentially the most helpful is WolframAlpha, as Ami said, you can use your browser history here too.
WolframAlpha can carry out complex equations can comparisons much like a TI Calculator. Additionally they have some areas where you can see the simplification of an equation paired with charts and graphs where possible.
A: Some people have suggested Sage or Python. Sage can do the symbolic manipulation that TI calculators can do using Maxima. Python with the right packages can do this as well (SymPy package I believe). Plus these things will be programmable. Sage however is fairly heavy at a few GB if I recall correctly. I'm not sure if there is anyway to interface with Maxima through Python like in Sage without actually having Sage.
A: If I am at school (and in my lab) I will just have Maple running for symbolic stuff. It's pretty good at it. Other then that, for simple calculations I use windows calculator (its a hotkey on my keyboard) and for more advanced stuff, I'll use WolframAlpha.
However, my plan is to learn mathematica. 
A: Here are three links you may find useful:

*

*WWW Interactive Multipurpose Server


*Some BCMath/PHP number theory programs


*Number Theory
A: For those of us on a Mac, there's Soulver, which is more like a notepad with calculating in mind. Lots of interesting features like conversions and instant stock prices, but no graphing if you're reliant on that.
If you're interested in having a history of disconnected equations over the course of a day, you could just open a new document, use it for the day and then discard it when you quit. However, it's more designed to create a set of calculations which you can save as a document and then return to later. You can link calculations together as well, letting you change a single value and recomputing all your answers instantly.
A: I just discovered a software called SpaceTime (now called MathStudio). Its free, available for Windows/Mac/iPhone and very powerful. You can:

Graph 2D, 3D and 4D function,
parametric, polar, vector fields,
implicit, contour, and fractal plots
with the tap of a stylus or click of a
mouse.  Move, zoom, rotate and trace
graphs in real-time.

Furthermore, it supports writing scripts.
The UI is good, and the software comes with a great tutorial.

A: I prefer MATLAB for heavy computing....And Wolfram alpha for quick references...both of these are extremely reliable....
A: Personally, I always use Python's console.
It has history and allows all kinds of math operations.

It is available for Linux, Windows, Mac, ChromeOS, Android, and others.
A: A few people have mentioned Sage, but I think deserves a bit more of a plug. Firstly someone mentioned that Sage is quite large to install. This is true, the binary download comes in at around 1 gig, however you can use it for free online, without having to install a thing! Head on over to The Sage Notebook, and sign up for a free account, which will allow you to create and save your own worksheets. I think this is really nice--it feels like it's your "gmail" but for maths, so you can just log in from anywhere and "check your maths" if you want :-D
This is a very popular option--I believe that in the last 9  months around 30,000 people have created accounts! It's perfectly suited for high school students and undergraduates, since no installation is necessary, and hence it can be used straight away in any computer lab with internet access. Here's a screenshot of what the notebook looks like. (I just googled "sage notebook screenshot.") It's been designed to look and work like the Mathematica notebook system, and it's surprisingly slick and easy to use.

There really is a lot to recommend it. I used the online notebook for around 6 months before I installed a copy on my own computer, and this was because I became interested in developing Sage. Although Sage is based on Python (so in particular any Python syntax will work), you really don't need to know Python to get started using Sage--I didn't when I started.
Sage is quite powerful too. Aside from using Maxima for symbolic integration, I belive Sage uses Pari for highly optimised polynomial arithmatic, and I have used Sage for some pretty intense linear algebra computations (over the integers as well as over finite fields), although for this kind of thing you'll probably not want to rely on the online server which can be a little slow.
And let's not forget that you also get the moral boost of being part of something really positive--an movement to create an open source alternative to costly and "closed" alternatives like Mathematica, Maple and Matlab.
(Disclaimer: I just came back from the Sage Days 23 sage developers workshop in Leiden, and I'm a bit pumped!)
A: Google's calculator is very powerful:
Google Guide Quick Reference: Google Calculator (Cheat Sheet) and your use history will be stored in your browser history.
A: SpeQ is really nice. If you close it's left/right areas it's just like Notepad, but a Notepad which can compute stuff. You can change any line displayed, press Enter at it's end and the line will be recomputed.
Just like in Notepad, you can select any part of what's displayed and delete it. You can also paste non-mathematical text into it. It will also remember what's displayed inside it the next time you start it. It has an option to clear it's content on Exit.
A lot of people recommended Python. It's is my main programming language and I love it, but SpeQ it's MUCH more powerful and easy to use than Python as a calculator.

SpeQ Mathematics
A: I tend to use Mathematica heavily for its symbolic capabilities, but I have a soft spot for Octave, an open-source MATLAB clone. For simple number crunching, it works nicely.
A: If you want a quick RPN calculator with print capabilities for standard calculations, I use Free42 which is compatible with HP42S (RPN). There is a print key to print when you need and you can configure the file used to save those prints. You can use skins to make it look the way you want, including the original HP calculator.
Free42 : An HP-42S Calculator Simulator
A: I recommend the free and open-source Sage software. Its a Computer Algebra System and a competitor to all the expensive over-the-counter CAS programs you can get. Fully functional.
You can also go to sagemath.com for an online hosted equivalent.
There is also Maxima, free for download. R and R-Studio are free also, if you are specifically gearing toward statistics. As well as Python and its numerous mathematical packages, also free.
If youre a student, you might get Matlab or Mathematica for free from your university, or at discount.  
A: insect is a web-based scientific calculator with full support for physical units:

A: maple or maplemathlab are pretty good at what you need it to do 
