Every good party needs music! Be it a reception or a formal dinner party if people stop talking for a moment the room shouldn't go SILENT. However their a different rules for parties with dancing or parties with mingling.
First parties with dancing: A good mix is a necessity. My rule of thumb is 3 fast songs 1 slow song. However if you would like a faster party then you can increase that ratio to up to 10 fast songs 1 slow song. I think that ratio only works for a group that loves dancing and is fairly young but if that is the crowd you are catering to, go for it. However don't just listen to the tempo, think about the beat. The reason "club music" like Usher and Justin Timberlake is so popular is because of the beat, it's danceable. You want music with a nice steady beat preferably in a 3 step, 4 step, or 2 step. Think of Latin music that lends itself to salsa (4 step), swing (3 step), or club music with a 2 step. Even slow songs should have a similar beat just a slower tempo. Be sure you listen to the lyrics, after all you really shouldn't have a break up song at your wedding reception. Some of my favorite party songs: "Say it Ain't So" by Weezer, "Someday" by Sugar Ray, "Earth Angel" by the Penguins, "Waking up in Vegas" by Katy Perry.
Second parties without dancing: They still need music! It should be easily talked over, fit the theme, and not distracting. I like instrumental Disney songs but that's because my friends and I are Disney nuts. I also enjoy instrumental show tunes like "Come What May" from Moulin Rouge and "All I Ask of You" from The Phantom of the Opera. Classical music can make a party feel more formal and techno beats can add to a contemporary vibe. Use the music to add to the theme (the soundtrack to Lord of the Rings for a fantasy feel, Disney music and the soundtrack to Pirates of the Caribbean for a Pirates and Princesses party) and your party will feel even more put together. On Youtube user CalikoKat posts great piano versions of show tunes and Disney songs among others.
Generally DJ's are a good idea for very large parties like weddings and dances but you don't need an expensive sound system or to hire a DJ to get music for your party. Make a mix CD or two (be sure to make it as long as you think your party will last and put it on repeat in case your party runs long) and put it in a boom box or make a playlist on your MP3 player and hook it up to speakers (sold at Target, Wal-mart, Best Buy, and a billion places online for as little as 20 dollars. For important events you can even back up you playlist on a friend's ipod and have her's standing by if there is a problem with yours.
~Adrianna