Generally considered the riskiest type of domestic bond, bonds issued by corporations, as opposed to government entities, typically offer the highest interest payments. Those bonds with the lowest credit quality ratings (BB and below) are considered "junk" bonds.
For diversification beyond the U.S. and exposure to foreign currencies, world bond funds have been attracting investor assets recently. Such funds usually invest the lion's share of assets in bonds issued by foreign governments, but they may also hold bonds issued by foreign corporations. Emerging-markets bond funds have historically been considered even riskier, but they have been gaining a lot of attention recently, too, as their yields are often higher than bonds from U.S. issuers.