Great for hotels but this now has many apartments such as those found on AirBnb. This website tells you up front what the total cost of the stay is rather than clicking through to find there is a 70 Euro cleaning charge that was not displayed initially.
Read the reviews not just the star rating before booking. What bothers one person may be nothing to you (such as a bad review for no booze in the mini-bar).
I also use this site to find the best price for a next night booking. After doing my research I will try my best to contact the hotel directly and will book directly with the hotel if they match or better the price. All of the booking sites take quite a percentage so you can help out owners by booking directly.