This one can catch many people off guard. You haven't been doing any spamming, spammy link building and you haven't been hacked but Google treats you as a spammer.
If you have the spammy host penalty it's because your website is hosted on the same servers as other sites which are spamming and Google has decided, because of the scale of the issue, to just penalize all the websites on that server or with that hosting company.
This is most common with services where you can host a website for free unless they have very careful procedures in place. Free hosting is a magnet for spammers who are looking to set up shop for very short periods of time before moving on.
If you are serious about your business or website then secure hosting on from respectable hosting companies can be found for a few dollars a month and give you peace of mind over what your neighbours are doing.
These hosts actively monitor websites they host for spam like activity – such as sudden bursts of activity where hundreds of thousands of emails are sent out - and often take these sites down themselves before Google becomes involved.
Again my personal recommendation is Webfusion but there are plenty of options out there including Hostgator and GoDaddy.
If you have been hit move your website to a better host and then submit your reconsideration request to Google.
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.