Here is a list of my favorite and what I consider the best wordpress plugins available.
Antispam Bee by Sergej Muller
AVH Extended Categories Widget by Peter van der Does
Bad Behavior by Michael Hampton
Better WP Security by Bit51
Contact Form DB by Michael Simpson
Drop Shadow Boxes by Steven Henty
Fast Secure Contact Form by Mike Challis
Ozh’ Admin Drop Down Menu by Ozh
PC Custom CSS by Peter Coughlin
Show Hide Author by Michael Spyratos
Slideshow by StefanBoonstra
Swerve by MKDO Ltd.
TW Disable Revisions by Igor Vuckovic
TW Recent Posts Widget by Igor Vuckovic
Widget Logic by Alan Trewartha
WP-PluginsUsed by Lester GamerZ Chan
XYZ WP Newsletter by xyzscripts.com