Most of the time when I'm out of the house, I have in my pocket a copy of the Summa Theologiae (English), the Opera Omnia of St. Thomas Aquinas (Latin), the complete works of John Henry Newman, all of Plato (Hamilton and Cairnes) and Aristotle (Barnes), Wittgenstein's Collected Works, the Church Councils up to Vatican II, Descartes' Meditations (Latin, French and English), Spinoza's Ethics (English), St. Anselm's Cur Deus Homo (English), seven volumes of Kierkegaard's Journals and Papers (English), the Gerhardt edition of Leibniz (French and Latin), 49 volumes of Harvard Classics, the Jesuit Relations, Hume's Dialogues, Enquiry and Treatise, several texts by John Paul II and Benedict XVI, the Catechism of the Catholic Church, the 1913 Catholic Encyclopedia, the Matthew and Mark portions of the Catena Aurea, Darwin's Descent and Origin, the Iliad and the Odyssey, Pascal's Pensees (French), Malebranche's Dialogues (English), Dal's Russian Dictionary, a 17th and a 19th century French-French dictionary, a Latin-English lexicon, a dictionary based on WordNet 3.0, the 1913 Webster's, Strong's lexicon, the Old Testament (Hebrew, Greek, Latin, JPS, RSV and KJV), the New Testament (Greek, Syriac, Latin, RSV and KJV), the Liturgy of the Hours, a bunch of fun science fiction, the stories of Saki, some children's literature to read to my daughter, typically part of an audio book (mainly from librivox) that I listen to while exercising or doing dishes, and various other items (including an out-of-date copy of my own website so I can refer to my own papers).
At conferences and in academic discussions, I typically have most of the primary texts that interest me right in my pocket, most of them in searchable form. It can be rather fun to pull out an apposite text from a primary source and quote at a conference. Granted, some of the translations are dated, and some of the texts are not the latest editions, but they are all very much usable.
Technical note: I keep all the texts, except the ones that are only temporary, on one 1gb SD card (they now can be had for less than $10) inside my Palm TX PDA. The Biblical texts are free, using the PalmBible+ format. The other texts are all in Plucker format (the latest and most reliable PalmOS version is here). The Biblical and Plucker texts I tend to view using a smooth and readable Utopia 16 font. The dictionaries are mostly home made and in RoadLingua format (RoadLingua is shareware; PalmBible+ and Plucker are free and open source). The Wittgenstein, Plato, Aristotle and Kierkegaard texts are authorized home-made conversions from rather expensive Past Masters databases. The Catholic Encyclopedia text is online, but I got it from the New Advent CD-ROM. The other texts cost me nothing, except for time. The Plucker ones are typically conversions from the web (using Plucker Desktop, Sunrise, JPluck or SunriseXP). There one needs to be careful of copyright. In cases where there is little added creativity in the html file over and beyond an original public domain file, I do not worry about copyright. In some cases, I write the web site administrator for permission to convert the texts for personal and/or scholarly purposes, and I almost always get the permission. In some cases, I only download files temporarily for the purposes of reading them and delete them afterwards, assuming (whether correctly or not only a court can tell) that this is sufficiently similar to the use of a VCR to constitute fair use. Some web site administrators, very conveniently, give clear and permissive terms for use of their texts and I am grateful to them. Worrying about the copyright issue is a nuisance.
I am kind of surprised that I know no other philosopher to carry a lot of ebooks on a portable device. I don't quite know exactly why. Granted, some of the texts took significant effort to convert, but a lot of them were very easy to do, there are a lot of pre-converted texts available in a variety of formats, and academics anyway do put in significant effort buying books, etc. The four NLX commercial texts were quite expensive (but then for the price I also got some very nicely searchable PC versions), but the other texts were cheap (Catholic Encyclopedia) or free (the rest). There is an investment in the hardware (in my case, the Palm TX), but the cost of the hardware is equal to the price of 5-10 books, and if one can get the books for free, it will pay for the hardware. I know some folks don't like to read texts in electronic formats, despite advantages such as convenient dictionaries, searching, hyperlinking, bookmarking and notetaking. The fact that less text fits on the TX's screen than on a printed page is a nuisance. But one gets used to it (I do most of my fun reading on the TX), and for quick reference one can easily tolerate it.