PopClip (Pilotmoon Software) was first released in July 2011 and proved to be an incredibly useful little piece of software. Since then it has become an indispensable piece of software, currently supporting over 100 extensions. The developer (quite rightly in my opinion) describes it as the Swiss Army Knife of Mac software.
PopClip basically takes a piece of text that you've highlighted with your mouse, pops up its little menu bar and lets you do something useful with it. Initially it was a mechanism for copying and pasting text, but extensions allow you to translate text, tweet it, convert it to a Bitly address (for URLs), create an email, send it to Facebook. Upto 22 extensions are permitted at any one time.
PopClip is available from the MacAppStore for £2.99 ($4.99 in the US) or you can download a free trial from the developers web site. I wouldn't bother with the trial - just buy it. You won't regret it if you do anything constructive with your Mac which is why PopClip has earned its place as the Apple Harvest's third Fruity Pick.
PopClip is supported on machines running OS X 10.6 and higher. The current version is 1.4.10 and works fine on 10.9.2. Extensions can be found on this page.