ShareX is an open source screen capture tool with a very extensive feature list.
The program doesn't just grab individual windows and a full screen, for example. You can also capture rectangles, ellipses, triangles, diamonds, polygons or freehand shapes. There's support for multi-monitor systems, and even a screen capture tool to save desktop activity as an AVI or animated GIF.
Any image you might grab can be processed in many ways. You can crop, resize or rotate it; annotate it with lines, arrows, shapes, textboxes and more; obscure confidential details (passwords, say) with the obfuscator; customise it with a watermark, or apply special effects, including borders, shadows, torn edges and more.
Once you're happy, ShareX can directly upload your work to more than 30 online services, including Twitter, Flickr, Picasa, Photobucket, Dropbox, RapidShare and GoogleDrive. You get a module to create custom uploaders for sites which aren't in the list. And there are also more general options, like the ability to send a grab via email, or copy it to a shared folder.
Even now, the program hasn't finished. Once the capture has been uploaded, you can have ShareX do something with the link: use your preferred URL shortening service, post it on Twitter, send it in an email, maybe just paste it to the clipboard.
Best of all, this is extremely configurable. You might set your preferred file format, set up the effects you'd like to apply, define a precise format for the saved file name, launch a custom program to work on it, select multiple upload destinations (you're not restricted to just one) and more, then launch the entire process with a custom hotkey.