A vst-dx adapter allows you to run vst plugs in a dx application, such as Sonar.
Fxpansion has a good one. Cakewalk bought it, if I'm not mistaken, so it should be on their website.
Get it, and you can use plugs like Amplitube (awesome) and The Fish Fillets (free), which includes the Spitfish de-esser, the Floorfish gate/expander, and Blockfish, a neat compressor that actually enjoys squashing stuff.