As fond as I am of Ovid’s treatment of Liberalia in the Fasti, I think Augustine’s description below really takes the cake:

Now as to the rites of Liber, whom they have set over liquid seeds, and therefore not only over the liquors of fruits, among which wine holds, so to speak, the primacy, but also over the seeds of animals:— as to these rites, I am unwilling to undertake to show to what excess of turpitude they had reached, because that would entail a lengthened discourse, though I am not unwilling to do so as a demonstration of the proud stupidity of those who practice them. Varro says that certain rites of Liber were celebrated in Italy which were of such unrestrained wickedness that the shameful parts of the male were worshipped at crossroads in his honour. Nor was this abomination transacted in secret that some regard at least might be paid to modesty, but was openly and wantonly displayed. 

For during the festival of Liber this obscene member, placed on a little wagon, was first exhibited with great honour at the crossroads in the countryside, and then conveyed into the city itself. But in the town of Lavinium a whole month was devoted to Liber alone, during the days of which all the people gave themselves up to the must dissolute conversation, until that member had been carried through the forum and brought to rest in its own place; on which unseemly member it was necessary that the most honorable matron should place a wreath in the presence of all the people. Thus, forsooth, was the God Liber to be appeased in order for the growth of seeds. Thus was enchantment (fascinatio) to be driven away from fields, even by a matron’s being compelled to do in public what not even a harlot ought to be permitted to do in a theatre, if there were matrons among the spectators. (De Civitate Dei 7.21)

And that cake (or liba) is penis-shaped, just like the giant phalloi that were trotted around the city and countryside in their little wagons, imbuing the land with fertility and driving off winter sterility and malignant enchantment.

This naturally reminds one of similar Bacchic rites carried out during Anthesteria and Dionysia, but also of a Vanic ceremony recorded in Chapter 40 of Tacitus’ Germania:

The Langobardi, by contrast, are distinguished by the fewness of their numbers. Ringed round as they are by many mighty peoples, they find safety not in obsequiousness but in battle and its perils. After them come the Reudingi, Aviones, Anglii, Varini, Eudoses, Suarini and Nuitones, behind their ramparts of rivers and woods. There is nothing noteworthy about these peoples individually, but they are distinguished by a common worship of Nerthus, or Mother Earth. They believe that she interests herself in human affairs and rides among their peoples. In an island of the Ocean stands a sacred grove, and in the grove a consecrated cart, draped with cloth, which none but the priest may touch. The priest perceives the presence of the Goddess in this holy of holies and attends her, in deepest reverence, as her cart is drawn by heifers. Then follow days of rejoicing and merry-making in every place that she deigns to visit and be entertained. No one goes to war, no one takes up arms; every object of iron is locked away; then, and only then, are peace and quiet known and loved, until the priest again restores the Goddess to her temple, when she has had her fill of human company. After that the cart, the cloth and, if you care to believe it, the Goddess herself are washed in clean in a secluded lake. 

Then there’s the Völsi, a magically preserved horse’s penis that was used in household cultus. You can read the original account from the Völsa þáttr, as well as some potent analysis here (which goes into some of the herbs that may have been used to preserve the phallos, among other fascinating details) and this page, which also has some pics of the sacred dicks and related ritual implements – which, like the tools in Liber’s ceremony, are primarily handled by respected, pious Matrons.