One of the most popular services for sending free texts to a cellphone from a PC is Txt2day. No registration or software downloads are required to use the service.
One disadvantage of Txt2day is that you must know the recipient's wireless carrier. If the recipient's wireless carrier is unknown, TxtDrop is a good alternative that does not require this information. Both websites allow replies to be sent to an e-mail address you specify. Similar providers include TextEm.net and SMSEverywhere.com. Alternatively, you can send texts to a cellphone over email. For example, if the intended recipient has the phone number 555-867-5309 and AT&T as his wireless carrier, you can text him by sending an email to email@example.com. A full list of wireless carriers' email addresses is available at ComputerHope.com