The United Club costs $550 a year, and gets you access to 45 locations worldwide, as well as select Star Alliance lounges. A day pass here runs $55.
Alaska Airlines' Board Room has locations in Seattle, Portland, LAX, and Anchorage, and costs $450 a year. Four locations might sound pretty limiting, but Board Room members also get access to most domestic Admirals Club locations for the member and their spouse, or up to two guests. If you don't fly much internationally, this is a pretty good hack that'll save you $100 over what American charges. If you live in Alaska, it's a no-brainer.
The granddaddy of them all is the American Express Centurion Lounge. AmEx Centurion cardholders get access to over 1,000 lounges worldwide, but the seven actual Centurion sites (in Las Vegas, DFW, Miami, LaGuardia, SFO, Seattle, and Houston) blow them all away. Most feature all the amenities we've listed above, plus spa services and food crafted by top chefs. Scott Conant in Vegas and Michelle Bernstein in Miami, to drop a couple names.