I have been using my TI-84 Plus Silver Edition. It is very useful for such a simple device. The TI-84 Plus Silver Edition has a 16 MHZ Zilog CPU, 24 KB of usable RAM (it has 128 KB but you can only use 24), a USB port for fast transfer and a serial analog port for backwards compatability with the TI-83 series about 1.5 MB of FlashROM (I think it has a little more, but that is taken up by the TI-OS), and It is fully compatable with the TI-83. Despite these weak system specs (it sorta resembles the 1980's era PCs, such as the Commodore 64, Amiga, and Apple II. It is programmable as well. TI calculators have a strange variant of BASIC (TI-BASIC) built in. You can program it. It can also run assembly apps.
There are several cool apps for the TI83/84 series. To install them you either need the TI Connect or a program called TiLP (TI Link Program) if you use Linux, BSD, or would prefer to use TilP's explorer. You also will need a USB cable for the TI-84 or a serial USB cable for the TI-83. You can also get USB and Serial calculator to calculator cables to share files with friends. If you have a TI-83 Plus, TI-83 Plus Silver Edition, TI-84 Plus, or TI-84 Plus Silver Edition (the origional TI-83 lacked flashable ROM.) you can update the OS on your device. Go to 2nd + (mem) and hit about. If you have a TI-83 Plus series it should say you have version 1.19, if you have an 84 it should say 2.41 (this is current as of this post being written). If it doesn't use TI device explorer to update the OS using the updater or download the latest from TI's Calculator website. Updating the OS will clear the RAM, so back up anything you need. The updater might find updates for other stuff too. If your TI came with apps in other languages, bring up the MEM menu I mentioned earlier and hit apps and then go down to the unneeded other language version of an app and hit del. This clears up space. Also if there is stuff you don't need, you can delete it too. It is good to check every few months for OS Updates. Updates add features, improve stability, or improve performance.
If you want to add any apps you can find tons of TI official apps at TI's Website. There are some useful apps for the TI calculators. Many require a copy of MirageOS. Mirage isn't really an OS, it is actually a shell. It provides an API for your TI. Copy Mirage to your flash. You can use the Alpha key while in mirage to change settings (notably you can turn on lower case letters. To use lowercase hit alpha twice. You can find MirageOS software at the Archives. Another must have is Graph3D. Once you run it you can do 3-D graphs by hitting Y= twice. You can find loads of TI software at ticalc a website devoted to the TI calculators.
I would back up your files after your done installing stuff. It is kinda amazing how much fits on a tiny amount of space.
One interesting app to keep an eye on is the USB Driver for the TI-84. The goal is to allow the the TI-84 to plug in USB devices such as a USB mouse, USB keyboard, and flash drives. It is not really stable yet so I would wait for a while before using it. I think the nicest thing would be to be able to copy apps and programs to and from the flash drive.