Drawloop will allow you to select which attachments you want to send, but the catch is you have to send a PDF merge file as part of the email. But if that is what you are doing anyway then Drawloop has some amazing features for attachments including watermarking them.
Conga will let you add an attachment via a query, and just send an email, so as long as you can query the attachment, this app is probably better for this one scenario.
If you are sending from cases, email to case premium will do it also.
Whilst I've personally used all those options, there is also this which seems to do the job but it's exy when you can use one of the other apps that also does so much more.