Google Transliterate has been available for a while, but what’s nice is that it now supports Sinhalese too. Check it out:
If you type “Roshan”, it’ll transcribe exactly as it should. My surname is a bit trickier so you can’t type it as it’s normally written in English but you instead have to type “sembakuttiarachchi” to get the correct transliteration.
Windows Vista and Windows 7 come with Sinhala support out-of-the-box. Windows XP doesn’t initially support Sinhala, but there’s an extra language-pack you can install to make things work. Once you install the correct fonts, things work properly on Linux too, but it’s a different story with Mac OS X. Even with the latest version of Snow Leopard, Mac OS X has problems rendering Sinhala properly.