I've recently started using two little applications on Mac OS X that I have already found quite helpful: PopClip and Liquid. Both allow you to do different things with selected text, and both are under $5.
When you select text in any application, a pop up menu comes up much like you find in ios. The menu shows different things you can do with the text. There are some options baked in, but where it really shines is when you add popclip extensions. There are options to add reminders, open a link, send an email, and other helpful actions. Between the extensions listed on the popclip website and Brett Terpstra's popclip extensions, there are all sorts of useful things for writing in markdown, like quickly adding the first link google fetches (like I just did with to make the link on the word 'markdown'), making lists, wrapping selected text in quotes or asterisks, or indenting or outdenting the selection.
Liquid duplicates some of what Popclip does, and unless you need the extras you might find Popclip enough for you. Liquid works by selecting text and triggering with cmd-shift-2. Then you can do all sorts of things with that text through easy keyboard shortcuts. You can search for the text in google, or set up custom searches (I have one for google scholar and one for the amazon kindle store). You can also translate the text to/from just about any language, which I use fairly often (despite the inelegance of internet translation). What I think might be top use is for researchers is the 'copy sentences with' option. For example, I could highlight the text of an entire article or book and have Liquid copy all the sentences with a specific word or phrase – very handy. You can also trigger Liquid without selecting anything and pressing option-space and typing your own text. It also functions as a calculator.
There is much more information on the websites of the applications, but I've tried to show a couple ways I have found these apps useful.