Well, guerilla marketing is still used a lot in the advertising industry. Some people describe it as actions undertook by small businesses to achieve a lot with small means... but in reality Guerilla marketing has many synonyms (buzz marketing, viral marketing, etc) and could be considered as any actions made by anyone using a small budget and aiming to have a big impact (such as with a word-of-mouth component).

As for it's effectiveness... I can't say how useful it is for small sites as I mostly use it for major corporations with large budget and a varied mix-media. But if you want some ideas of what could be done don't hesitate to send me a pm.

Frank