http://www.textmefree.com lists over 200 and reviews some of them.
Some reviews may be a bit out of date, and one or two services may no longer exist, but it's useful as a starting point for some to try.
Take care to read the descriptions and terms though, as just a few of these deliver the message as premium rate to the receiver.
Cbfsms is good for UK, and you'll see ads for their paid service Fishtext, which you might consider, about 1 or 2 pence a message to most countries. It simulates their own number, or you can buy one of theirs then log in to read them.