The paid version lets you split and merge iPhoto Libraries while keeping all of your meta data intact. Meta data is all the information about your photos such as faces, keywords, star ratings, albums, slideshows and keepsakes. You can split and merge iPhoto Libraries from within iPhoto itself using Export and Import but you lose all the meta data associated with the photos.
The paid version of iPhoto Library Manager costs $US20 and is available here.
The iPhoto Library Manager website also describes additional features of IPLM not mentioned here as well as help and support forums.