You've most likely seen this MANY times: "Contact me today for your free CMA!"
A Current Market Assessment is the most popular method for an agent to come up with a realistic market value of your home. We'll consider the neighbourhood, the type of home you have, current listings in your area, and recently sold homes among other things.
In my opinion, recent sales history is the most effective way to come up with an accurate value.
Seeing the neighbours house down the street listed at $30k over its value is not really helpful other than making yours look more attractive if you do list. When you think about it, anyone can ask whatever they like for their house, but at the end of the day, it's only worth what someone will pay for it. So, seeing actual sales that have closed will give you a much more accurate picture. I like to use the shortest amount of time I can to give me a decent amount of data. These days, in a typical neighbourhood, I can usually get good info using a 90 day history, but I'll stretch it to 180 days if I have to.
For residential homes, some of the things I take in to account when looking at "comps" as they like to say on HGTV are:
The age of the home, property size, is there a garage?, condition of the roof and windows, square footage, number of bedrooms, number of bathrooms, is it up to date?, does it have a basement and if so, is it finished? electrical system and heating system.
Have you had someone give you a CMA? Was the value what you expected?