The very first 3 3/4 inch astromech droid (R2-D2) was released in the 1978 Star Wars Early Bird Kit. Kenner/Hasbro and other licensees have been releasing astromech droids continously ever since.
Some of the more obscure astromech doids included R2-D2 with antlers or R5-D4 which came with a rocket launcher inside the body. Hasbro has done a very good job keeping astromech droids interesting, some figures had electronics built in, others had battle damage or panels which could be opened.
Some very popular 3 3/4 inch astromech droids are the Disney Theme Park exclusive special occassion astromech droids, such as the Halloween astromech droid R2-B00 or the Holiday 2015 droid R2-H15. Besides Disney's themed astromech droids, they've also been releasing build a droid parts since Spring 2012 (before Disney bought Lucasfilm) in Disney parks where people can build their own astromech droids.