VLC Media Player: hands down, the best media player out there.
Google Desktop: blows standard windows search and even the new one out of the water. Ridiculously fast.
WinAce: versatile, easy to use zipping software. .ace is one of the most compressed forms out there, and it also works with more standard rar and zip files.
uTorrent: very streamlined, easy-to-use torrent client. Takes up minimal hard drive and memory, and runs beautifully and dutifully in the background.
Please add your favorites as suggestions!