...and just to stir the pot a little more, I'll offer my two
cents: Combination of PaintShop Pro and good ol' Photoshop.
Unfortunately, this is probably the most expensive solution, but --
at least with me -- it offers the most control. PSP is great in
that it allows you to configure your screen captures: You can set
it to capture the full screen, or just the foremost window, set up
for single or multiple captures, include the cursor in the capture,
or leave it out, etc. I'll then just use the Marquis Tool in PSP to
select specifically what I want and copy it into a new window in
Photoshop; where I can add any effect I want, work in layers, add
text, etc. (it's fantastic in Photoshop if you have access to a
font that matches whats in your UI, combined with layers you can
rig your screenshots to match scenarios that you may not be able to
actually set up from the local instance of the UI your
documenting). Now I'm sure that SnagIt may offer similar tools, but
I can't speak for it, because I haven't used it.
On another quick note, if your UI is web-based, Firefox
offers a great new screenshotting tool that is built in to the
browser (FireShot). It allows you to capture the full browser
window's contents, even if it can't be displayed on screen (i.e.,
you have to scroll up/down). It's quick and easy, and allows you to
directly integrate with your image editing application -- click on
a browser button, and the screenshot will automatically open in
your editor app. And best of all, IT'S FREE!! Check it out if you
want...
http://screenshot-program.com/fireshot/
Good luck!