there are plenty how big is your best site? if it is up to 2000 pages just look for "free xml sitemap" and you will find the one for you.
What kind of site is it? If it's a CMS (Content Management System) there's loads of effective plugins that can help you out. However, if it's a static HTML design or something similar, YSEO's advice is probably your best bet
Remember that you can submit your site map to more than just Google. Ask, MSN, and Yahoo also support sitemaps.