The SharpDevelop project switched licenses with the 2.0 release in December from GPL to LGPL. That means you can use the components in own applications, both open and closed source. Unlike the GPL, you can statically or dynamically link to LGPL code from open or closed source applications as long as you don't make any changes to the LGPL'd code. This goes against the goals of the Free Software Foundation, but he who writes the code picks the license.
The SharpDevelop switch to LGPL is good news, because SharpDevelop contains some advanced, mature code. Especially useful are the Core / Addin system and the text editor component (which was the cause of this fracas 3 years ago).
The SharpZipLib remains GPL'd but with exception clauses that let you use the component (not the code) anywhere - excerpt from the site: "Bottom line In plain English this means you can use this library in commercial closed-source applications."