SharpDevelop is quite good, but at my company we've found VBConversions to provide a much more complete conversion. It's a commerical app though, but for the time saved over SharpDevelop it was a no-brainer for us.
As a specific example, one thing we found that SharpDevelop didn't convert correctly was VB indexes, which use curvy brackets. It seemed unable to distinguish between indexes and method calls so didn't convert the indexes to square brackets. VBConversions converted them fine. This one thing made it worth its purchase for us.
It looks like the Tangible Software product works very similarly to VBConversions.
