The greatest App there is right now is smshare
You have to install this app on your Android phone, and the smshare Extension on your Chrome Browser.
Then you can type your SMS in the Chrome-Extension, and then the SMS will be sent to your phone, which then sends them as normal SMS. No Wifi hassels, and really comfortable and fast.
A new feature of smshare is that your incoming sms can be forwarded to your email account - so you don't even need to touch your phone for communication via SMS.
Another way is to use the Wifi SMS Apps.
On these kind of Apps (like for example RemoteSMS) you have to connect your phone to the same Wlan as your Computer, start the App on the Phone, and then you can log into an Webinterface with your Browser and compose the SMS there which then are sent from your phone.