You can log into your sprint account from your PC, scroll all the way to the bottom where the yellow line separates it. There are captions like Learn About, Shop, Do Stuff. Go under Do Stuff and the second option is send text.
Click on "My Device & Media>Messaging" Or you can bookmark this url; http://sprint.zipwhip.com/ Or if it is another Sprint user you can send it from your email by addressing it to email@example.com Other carriers also have messaging from your email to phone address's you can google for those if you want.
Here's a site I had bookmarked, note that the thread is over 3 years old, so things my have changed:
Amp'd Mobile @vtext.com
Verizon @vtext.com (text)
Verizon @vzwpix.com (pictures)
US Cellular @email.uscc.net (text)
US Cellular @mms.uscc.net (pictures)
Cingular @cingularme.com (not sure)
Boost Mobile @myboostmobile.com
Einstein PCS @einsteinmms.com
But what if you're not a Sprint customer but need to send a text from your computer to a Sprint customer? I'd hate to have to walk down 4 flights of stairs and outside my building to get a signal every time I want to send a text to a Sprint customer while I'm at work. Verizon & AT&T let you send texts from their websites, so why doesn't Sprint? I can get the spam issue, but if you create a login for Sprint with your email account, there's still no way to do this -- unless if you're a Sprint customer. It's quite ridiculous if you ask me.
But what if you're not a Sprint customer but need to send a text from your computer to a Sprint customer?
You can send it from your email by addressing it to firstname.lastname@example.org