Merging two cells in multiple rows

1. Merge the two cells in the first row by selecting Home > Alignment > Merge Cells command.
2. Select the merged cell and select Home > Clipboard > Format Painter.
3. Click and hold the first cell on the next row, and select all the range by dragging your mouse. (For example, if you merged cells A1 and B1 in the previous step, now select range A2:B99.)
4. Excel applies the merge command individually to each row and you're done.

Excel has (had?) a Merge Across function. It's not on any of the toolbars by default, but is available as a customization. I usually add it to my Formatting toolbar, after the Merge Cells and Unmerge Cells buttons. (Right-click the toolbar, choose Customize, go to the Commands tab, select Format in the Categories list on the left, scroll down to find Merge Across, drag it to wherever you'd like.) Once it's on your toolbar, you can merge across however many cells with a single click. Important: Note that merging keeps the leftmost value only!

Unfortunately, Excel 2007/2010 has totally broken/gotten rid of the whole customization feature, so if you're using one of the newer versions, you're stuck doing the Format Painter thing.