Lately I have found Google Book Search to be very useful, especially in finding the texts of old Scottish songs. Google has digitized dozens of old collections of them. Of course, if you're looking for a particular song, the best way is still to search using a phrase from the lyrics, but I thought there might be people out there who'd like to browse in some of these books, so I compiled a list of them:

Abbotsford Series of the Scottish Poets, by George Eyre-Todd, 1896.

Ancient and Modern Scottish Songs, Heroic Ballads, Etc., by George Paton, 1869.

Ancient Scotish Melodies: From a Manuscript of the Reign of King James VI, by William Dauney, 1838.

Ballads in the Cumberland Dialect, by Robert Anderson, 1815.

Beauties of the Scottish Poets, or Harp of Renfrewshire, anonymous, 1821.

Early Scottish Melodies, by John Glen, 1900.

Gleanings of Scarce Old Ballads, by Peter Buchan, 1891.

Love Songs of Scotland: Jewels of the Tender Passion, by Robert W. Douglas, 1901.

Modern Scottish Poets: With Biographical and Critical Notices, by David Herschell Edwards, 1897.

Scotia's Bards, anonymous, 1856.

Scottish Song: Its Wealth, Wisdom, and Social Significance, by John Stuart Blackie, 1889.

Scottish Song, by Mary Carlyle Aitken, 1874.
Link fixed. --JoeClone, 9-Jun-07.

Studies in Folk Song and Popular Poetry, by Alfred Mason Williams, 1895.

The Amusing Songster, by Joseph Edwards Carpenter, 1866.

The Book of Scottish Poems: Ancient and Modern, by John Ross, 1878.

The Harp of Caledonia, by John Struthers, 1821.

The Harp of Perthshire, by Robert Ford, 1893.

The Harp of Stirlingshire, by William Harvey, 1897.

The Illustrated Book of Scottish Songs, anonymous, 1854.

The Leading Poets of Scotland, by Walter Jenkinson Kaye, 1891.

The Modern Scottish Minstrel, by Charles Rogers, 1856.

The Music of the Waters: A Collection of the Sailors' Chanties, or Working Songs of the Sea ..., by Laura Alexandrine Smith, 1888.

The Pictorial Book of Ancient Ballad Poetry of Great Britain,..., by Joseph S. Moore, 1853.

The Pocket Encyclopedia of Scottish, English, and Irish Songs, anonymous, 1816.

The Poets and Poetry of Scotland, by James Grant Wilson, 1875.

The Popular Songs of Scotland with Their Appropriate Melodies, by George Farquhar Graham, 1887.

The Scottish Ballads, by Robert Chambers, 1829.

The Scottish Minstrel: The Songs of Scotland Subsequent to Burns, by Charles Rogers, 1882.

The Scottish Songs, by Robert Chambers, 1829.

The Songs of England and Scotland, anonymous, 1835.

The Songs of Scotland, Ancient and Modern, by Allan Cunningham, 1825.

The Songstresses of Scotland, by Sarah Tytler, Jean I. Watson, 1871.

The Souvenir Minstrel, by Cornelius Soule Cartée, 1833.

The Universal Songster; Or, Museum of Mirth, by George Cruikshank, 1878.