Holdem Manager
Holdem Manager has become the main hand database software as of late. I actually made a switch to Holdem Manager after having already purchased PokerTracker 3. The software itself is just much more powerful, and I've also noticed that the HUD displays much more reliably and accurately than PokerTracker.
PokerStove
PokerStove is a great free program which can be used to figure out the odds of any number of hole and community card configurations. Check out the FAQ on the following link for help with using the program.
Mikogo
Mikogo is another free program which lets a user share their desktop with up to 10 other people. It makes it real easy to watch others play, or have someone else evaluate your game.
Skype
Skype is also a free program and it is my prefered choice for communicating with others via microphone over the internet. Skype is especially usefull when combined with Mikogo.