I have been using OS X since Novemberish 2007 and in that time I have found the following programs to be the most useful to me.

• Adium – General purpose chat client. Supports multiple protocols
• Deep Vacuum – Internet archiver and offline browser
• Firefox – The worlds best web browser
• Fugu – SFTP client
• EZ Money – Simple easy to use financial manager
• Disk Inventory – Graphical disk usage chart similar to kdirstat in Linux
• Handbrake – DVD Backup utility
• MacPorts – Package manager for installing Linux programs in OS X
• Perian – Extra video codecs for QuickTime
• Flip4Mac – WMV codec for QuickTime
• Transmission – Bittorrent client
• One Button FTP – FTP client
• Teleport – Software KVM for OS X
• TextMate – Text editor

Widgets

• Gas – Show the cheapest local gas price
• iStat Pro – Detailed system statistics
• WidgetTerm – Terminal

Screensavers

• RedPill – the Matrix
• Plasma Tunnel – Flying through a plasma tunnel. Great graphics