An increasingly more and more popular option is to work at festivals - after all, you get the chance to combine both work and play, experiencing all sorts of festival fun. Check out your chosen festival's homepage for more info. Or why not follow the festival? You could work on the bar, as gate security, on one of the food/clothing stalls, or even for a charity as a steward.
Another way of making the most of living abroad over the summer is to help out in the tourism and hospitality industry. Why not upload your CV at JustJobs4Students and search through their database of odd jobs across the world. The Summer Jobs website is also a pretty safe bet, with regularly updated pages and has a variety of jobs on offer, ranging from working a couple of weeks, to taking the entire couple of months of holidays to work abroad. You could have a look at a particular country, say working at Camp USA, or field - skiing in the Alps, Canada, Argentina? There are loads of chalets, hotels and holiday resorts (including camping sites) looking to hire over the peak summer months. Look into the Spanish Costa del Sol, the French Côte d'Azur, Grecian islands... Jobs range from helping out with room keeping, reception, administration and the odd repair work. Seasonal Jobs 365 offers up loads of areas to work in, with some particularly good postings for those interested in working in French châteaux.
If you're planning on going down the garçon de table route or flexing your arm muscle behind the bar whisking up cocktails, your best bet would be to find a town out in your chosen country that has quite an active social life. The more the bars and restaurants, the more likely you are to find a job quickly and easily. Many students just go out and find a job once there, but you could always check a few websites, such as Craig's List to see what's on the market, if you wanted to get an idea before going out there on a limb.
If you're looking to meet people from all sorts of different places around the globe, you might want to start thinking about working in a hostel. Jobs there are ten a penny, you get to meet some seriously interesting (and the inevitable odd slightly annoying) peeps and you might even be offered a room there in the process, so you'll be quids in! Trawl through sites such as Hostel World and Hostel Bookers, get emailing and throw in a few phonecalls and you're bound to find a job working at reception or behind the bar. Some hostels even offer tours of the city - you could ask if they needed anyone to lend a helping hand, get to learn about the town and its culture in the process, whilst then giving others the chance to get to know it too!
Aupairing is not for everyone, but if kids and living with a family seems like something you'd like to do, then make a note of AuPair.com or Great Aupair to find the ideal nuclear family. The great thing about working with children and within a family unit is that your language really does improve vastly, as you'll be crossing childish language with adult conversation, and you'll experience home cooking to boot! Some families also offer minor remuneration, so you might make a couple of quid whilst out there, too. Make sure you do a lot of research as families can sometimes be a bit hit or miss, but either way, you'll come back with a better grasp of the language and you'll have fun in the process. Even if it isn't exactly what you want to do in the long run, it'll only be for a couple of months - and you'll be speaking, eating and sleeping the lingo 24/7. Supremely beneficial come the following year abroad, or, if you're in your third year and studying more than one language, it's a good way to brush up on vocab and grammar from the first half of your year abroad.
More of an outdoors-y type of person? You can always get working in the open air, finding something on WWOOF, World Wide Opportunities on Organic Farms. It's an organisation that started in 1971 in the UK, and you can pretty much do any kind of work if you look hard enough, for varying periods of time. People offer up accommodation and food in return for your labour... and you'll not only improve your linguistic abilities, but you'll also get a tan if you pick wisely!
So all that's left to say is happy job harvesting!