The best resource available, as has been mentioned in another answer, is loungeguide.net. It's coverage ranges from excellent to OK, depending on the part of the world you're looking at.
It's worth keeping in mind that Airport lounges are not necessarily as good as you might have been led to believe. Some are excellent, including the services you've mentioned, such as showers, wifi, sufficient food to be considered a meal, alcohol, etc. However many do not have these services - showers are uncommon in many parts of the world (eg, very few lounges in the US have them), food is often at best snacks (one American Airlines lounge I was in recently had biscuits/cookies, and nothing else), Wifi is normally provided, but sometimes it's just the free airport wifi.
Before spending money to access a lounge make sure you're aware of the specific facilities offered. Also be careful with what you're actually buying. Some airlines (eg, American) will allow you to buy a "day pass" that can be used at multiple lounges on the one day - useful if you're passing through more than one airport in that day, whilst other airlines (eg, United) only sell a single-entry pass that can only be used once at a single airport.
Outside of the US, paid access to airline-run lounges is rare, however there are frequently independent lounges that will offer paid access, but prices and services offered can vary widely.
You could also look at Priority Pass, which gets access to multiple lounges around the world. Priority Pass generally requires an annual fee and/or per-entry fees, however it is also offered as a free extra with some credit cards (although in the US this will only get you access to limited lounges). Lounge Club also runs a similar but more limited service - not surprising given that it's run by the same company as Priority Pass!