I use sharebuilder and I like it. The costs are somewhat high ($12/ month) , but it allows me to split $100 per week into 5 different stocks... I like that.
Disclaimer: By providing links to other sites, FatWallet.com does not guarantee, approve or endorse the information or products available at these sites, nor does a link indicate any association with or endorsement by the linked site to FatWallet.com.