Services like Typecast make it much easier for designers to work with web fonts and I pretty much do all my typography for the web using it. It allows you to create typographical mock ups (just like you would in Photoshop) using its visual interface to assign fonts, styles, weights, colours, line height, margin, padding and more to your typography. The best thing about using Typecast is that it is all working within the browser so you are making decisions on type exactly as it will be rendered for the user. Does a certain font not look too hot at 19 pixels? Bump it up to 20 at a click of a button, back down to 19 before finally settling at 18 pixels in size, all done in seconds rather than hours sitting with a developer.