I would recommend not deleting anything out of that folder unless you are absolutely sure the application is no longer installed, like MisterMe said.
Be aware that there may be stuff in there that you think you don't need, but really do -- for example, stuff like "BSD.pkg" you may think you don't need anymore, but if that one gets deleted, you cannot repair permissions at all.
In addition, Software Update also checks this folder to see if you need an update or not to Mac OS X and various applications, which can cause problems if you remove the wrong receipt file.
My recommendation would be to leave that folder alone. 100MB is not a lot of space, considering Mac OS X takes about 3 - 4GB of space.